Orcs Must Die! Unchained (OMDU) is the third game in the tower defense meets third-person action game series, but in some ways it is a new beginning for the series. The core gameplay remains intact, but rather than charging you for the base game and then for a series of DLC expansions, OMDU has moved to a free-to-play model. The game is free to download on Steam or directly from the OMDU website and gives you complete access to all of the game modes for free right from the start.
The game uses a two-tier currency system, skulls and gold, with skulls awarded after every level based on your performance and gold coming at the cost of real world cash. Everything that you can buy with gold you can buy with skulls with the exception of vanity items, so the premium currency in the game exists primarily as a tax on your patience. That being said, don't expect to unlock all of the game's traps, heroes, and other such things after a couple of days. There will be grinding involved, but how much of a grind that you'll find that to be will depend on how much fun you're having playing the game. OMDU also shares some of the features of freemium games that will be familiar to those who've played those types of games in the past such as consecutive daily login bonuses, paid experience boosters, and discounted packages of gold or premium items.
As for the game, the basic concept will be recognizable to anyone who's played a tower defense game before. Enemies enter the level from one or more portals and then proceed to make their way to your rift, where they will exit. Let too many enemies exit and you'll fail the level. Survive all of the enemy attack waves with your rift intact and you win. Defeating enemies rewards you with currency which you can use to shore up your defenses between attack waves.